Rosin-free solder composition

ABSTRACT

A process for vapor phase soldering of electronic components comprises applying to the surface of a substrate or to the surface of components on such substrate a solder cream comprising 85 to 90% of finely divided solder metal and 10 to 15% of a fluorinated tertiary alkylamine, preferably a perfluorotrihexyl amine, by weight of the sum of the two components, and positioning electronic components on the substrate, with contacts of such components on the solder cream-coated surface. The substrate is placed in a chamber in contact with the boiling vapors of a second liquid fluorinated tertiary alkylamine having a lower boiling point than the fluorinated tertiary alkylamine in the solder cream, preferably perfluorotriamyl amine, in the chamber, and melting the solder metal in the solder cream and fusing the solder joints of the electronic components to form solder joints substantially free of organic residue. The fluorinated tertiary alkylamine in the solder cream dissolves in the lower boiling point fluorinated tertiary alkylamine in the chamber, and can be recovered from the solution by distillation for reuse.

This is a divisional of application Ser. No. 821,231, filed Jan. 22,1986 now U.S. Pat. No. 4,673,532.

BACKGROUND OF THE INVENTION

This invention relates to novel solder compositions or solder creams,and is particularly concerned with the provision of solder compositionsespecially adapted for soldering components in microelectronic circuits,and wherein the use of fluxes, particularly rosin type flux, is avoided.

A solder cream is a material that is capable of being applied to asubstrate or surface in a specific pattern using screening or analogousmethods which can subsequently undergo fusing to provide an electricaljoint or interface commonly described as a solder joint. The soldercream generally consists of metallic particles of various alloys such astin-lead, tin-lead-silver, etc., contained in a vehicle including a fluxsuch as wood rosin, or derivatives thereof.

Currently, microelectronic circuits are made by surface mounting chipcarriers and other components on ceramic and plastic substrates by vaporphase soldering. Circuits similar to those used on printed circuitboards are produced on the substrates. In certain instances, the circuitis located on one side of the substrate, with pads being provided incertain areas, other parts of the circuit being covered with aninsulating material.

The solder cream having a paste-like consistency is applied, for exampleby means of a silk screen, to the pads on the circuit board. Thereafter,the electronic components are carefully positioned with their peripheralcontacts on the solder cream-coated pads. When all of the components arethus in place, the board, with such components temporarily positionedand retained thereon by the solder cream, can be placed in a vaporreflow system and subjected to a sufficiently high temperature to causethe metal content of the solder cream to liquefy and the contacts of theelectronic components to be fused and to adhere to the pads on thecircuit board. Examples of solder compositions are described in U.S.Pat. Nos. 3,684,533 to Conwicke and 4,373,974 to Barajas.

However, solder creams presently utilized, such as those of the abovepatents, contain flux constituents which char during soldering and coatthe circuits with electrically conductive matter, causing shorts if notcompletely removed in post-soldering cleaning.

Thus, fluxes generally contain rosins which are difficult to clean fromsubstrates or circuit boards. If one attempts to reflow the solder usinginfra-red, the light frequencies are translucent to the flux, and tendto bake the rosin on the boards, making it extremely difficult to removeor clean the baked-on rosin residue. If vapor phase soldering is used toreflow the solfer, the tendency is to pre-bake the flux, creating thesame undesirable residue which is difficult to remove. Further, wheresolvents are employed to remove such residues, the solvent vapors arehazardous in work areas.

Since the reflow soldering process has many advantages such astemperature stability under inert atmosphere conditions, it was soughtto enhance the process to particularly avoid the above noted problemsassociated with deposition and removal of char residue from circuitboards. It was considered particularly desirable to replace the rosintype flux employed in solder creams used in electronic circuitry, with amaterial more compatible with certain liquid fluorinated compounds suchas perfluorotriamyl amine, often used in vapor phase soldering as thevapor reflow medium for heating the solder cream to liquefy same.

It is accordingly an object of the present invention to provide animproved solder composition or solder cream which is especially adaptedfor soldering components in electronic circuitry, particularlymicroelectronic circuits.

Another object is the provision of a solder composition of the abovetype which avoids formation of residues formed on substrates or circuitboards following vapor phase soldering operations.

A still further object of the invention is to provide an efficientsolder composition of the above type especially designed for use invapor phase soldering operations and which utilizes a vehicle for thesolder composition which is free of rosin or rosin type flux components,such vehicle being compatible particularly with certain fluorinatedcompounds employed as the heating medium in vapor phase soldering.

Yet another object is the provision of a vapor phase soldering processemploying an improved solder composition of the above type, inconjunction with certain fluorinated compounds employed to provide thevapor phase reflow medium.

SUMMARY OF THE INVENTION

It has now been found that the above objects and advantages can beachieved according to the invention by the provision of a solderingcomposition containing a major portion of finely divided solder metaland a minor portion of a fluorinated tertiary alkylamine which issemi-solid at room temperature and which confers a creamy consistency onthe composition at room temperature. A particularly advantageous aminefor purposes of the invention is a perfluorotrihexyl amine.

A solder composition or solder cream formulated according to the presentinvention contains no char-forming constituents such as rosins, and isespecially suitable for vapor phase soldering. Since no organic residuesar formed following soldering when employing the solder composition ofthe invention, no elaborate cleaning procedure is required to removechar residues from circuit boards after soldering. Further, asubstantial portion of the fluorinated amine component employed asvehicle in the solder composition of the invention can be reclaimed forreuse following vapor phase soldering.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The fluorinated tertiary alkylamine component functioning as vehicle forthe solder composition of the invention is a compound which issemi-solid at room temperature and has viscosity characteristics whichfacilitate mixing the solder metal with such vehicle at about roomtemperature and gives creaminess to the solder cream ollowing admixturewith the solder metal, for easy application, e.g. to pads on a circuitboard.

The fluorinated tertiary alkylamine is preferably a perfluorinatedtrialkyl amine containing from 4 to 8 carbon atoms. Most desirably, thematerial employed is perfluorotrihexyl amine, marketed as FC71 by 3M Co.This material is understood to comprise the straight chain and branchedchain isomers, and also mixtures thereof. This material has a boilingrange of about 250° to about 255° C., e.g. about 253° C. While beingsemi-solid at about room temperature, the FC71 commences to turn liquidat slightly elevated temperature of about 25° to about 30° C.

In addition to having the important advantage of not formingcarbonaceous residues on circuit boards or substrates following vaporphase soldering, the fluorinated tertiary alkylamine vehicle componentof the solder composition of the invention is compatible and completelymiscible with another related fluorinated tertiary alkylamine, mostdesirably perfluorotriamyl amine, having a lower boiling point than thefluorinated vehicle component and employed as the liquid medium which isvaporized in the vapor reflow system, to provide the heated vapor phasemedium during soldering. Thus, during vapor phase soldering, thefluorinated amine vehicle of the solder composition liquefies and drainsinto the liquid perfluorotriamyl amine liquid medium in the vapor phasesoldering machine and dissolves therein. Such medium containing thedissolved fluorinated vehicle, e.g., the perfluorotrihexyl amine, canthen be subjected to distillation to separate and distill off the lowerboiling perfluorotriamyl amine, and the recovered higher boilingfluorinated amine, e.g. FC71, can be reused.

The solder compositions of the invention contain finely divided soldermetals in the form of metallic balls or particles, dispersed in thevehicle. The solder metals can be any of the conventional single ormultiphase metals normally used for soldering, particularly tin, leadand silver, and alloys and mixtures thereof. Alloys or mixtures oftin-lead, tin-lead-silver, tin-silver, lead-silver and lead-tin, forexample, can be employed. A preferred mixture of solder metals employedin the solder composition of the invention consists essentially of amixture of lead, tin and silver, which can contain from 61.5 to 62.5%tin, 1.75 to 2.25% silver, and the balance lead. A particularlypreferred solder metal mixture consists of 36% lead, 62% tin and 2%silver. Another preferred solder metal consists of 63% lead and 37% tin.

The solder metal particles should be finely divided, preferably having aparticle size ranging from about 0.2 to about 100 microns, e.g. about 40to about 70 microns, in diameter. The metal particles are preferablyround rather than oval or irregular, since oval or irregular particlesclog the screens, whereas round particles pass freely through thescreen. Thus, with round particles proper flow of the metallic particlesthrough the screen takes place.

The two essential components of the solder composition of the invention,the finely divided solder metal and the fluorinated tertiary alkylaminevehicle, are employed in a range of proportions of 85 to 90% soldermetal and 10 to 15% of the fluorinated alkylamine, by weight of the sumof the two components. Since as noted above, the FC71 perfluorotrihexylamine material is comprised of one or more isomers, it has been foundthat the amount of such material employed to obtain the desiredviscosity characteristics can vary within the above noted 10 to 15%range, depending upon the particular isomers present.

Although the above defined fluorinated amine compound is the solevehicle component employed, in preferred practice minor amounts of othersubstances can be incorporated in the soldering composition of theinvention to provide improved efficiency in vapor phase solderingaccording to the invention. Thus, a small amount of an organicbrominated activator compound can be incorporated in the soldercomposition, which functions to react with and reduce any lead oxide andtin oxide in the solder metal powder during vapor phase soldering andconvert such oxides to the corresponding bromides, which can be washedaway from the solder joint to prevent contamination thereof. Thus, anorganic bromide such as bromocyclohexane or bromocamphor can be employedfor this purpose. The amount of such material employed can range, forexample, from about 0.1 to about 1% by weight of the total or overallsolder composition. A nonionic fluorinated alkylester, e.g. the materialmarketed as FC-430 or FC-431 by 3M Co., can be employed in the soldercomposition as a surfactant and scavenging agent to take up and washaway such lead and tin bromides.

A solvent such as isopropyl alcohol can also be added to disperse theactivator in the solder composition. Such solvent can also aid toachieve the desired viscosity for the solder composition, for example aviscosity of the order of about 500,000 centipoises at 20° C. The amountof solvent employed can range from about 2 to about 15 parts, e.g. 10parts, per part of activator, by weight.

The solder compositions of the invention are prepared by admixing thesolder metal and the fluorinated amine component and other optionalcomponents together in the ranges and amounts noted above.

Thus, for example a so-called dry box can be employed in which isdeposited a quantity of the vehicle consisting of fluorinated tertiaryalkylamine, particularly perfluorotrihexyl amine, according to theinvention. The metallic component of the solder cream can be in the formof a wire dispensed from a supply into a melting pot or crucible fromwhich molten metal alloy flows through a bottom orifice into a stream ofnitrogen gas and is reduced to small spherical particles ranging indiameter from about 0.2 to about 100 microns. The desired size metallicparticles are then dispensed in the vehicle, in the desired proportions,and are mixed with the vehicle by means of a suitable mixer apparatus.

While the above illustrates one method of forming and combining thedesired metallic particles with the vehicle to produce the solder cream,in practice, other similar methods may be used. This operationpreferably is carried out within an inert atmosphere, e.g., nitrogen, orthe like, substantially to aid in minimizing oxidation of the metallicparticles and to produce a solder cream substantially free of oxideswhich tend to inhibit attachment to the contacts of the chip carriersand the pads on the circuit boards, and to eliminate contaminated solderjoints.

The solder compositions of the invention can be employed for efficientlysoldering microelectronic components such as capacitors, resistors,integrated circuits and their packages or carriers, transistors, diodes,etc., onto a circuit, carried on a substrate, in vapor phase solderingoperations. The soldering compositions also can be used in automatedvapor phase soldering, e.g., for automobile assembly and electricalconnectors, and also can be used in automated assembly of jewelry andfor package sealing.

The solder composition of the invention can be applied to any auitablesubstrate such as metal pads on a circuit board to which contacts ofelectronic components are to be soldered. Such application of the soldercomposition or solder cream can be made by use of metal screening toapply the solder cream to the pads. However, other modes of applying thesolder cream in addition to screen printing can be employed, including,for example, dipping the objects to be soldered into the soldercomposition, or employing syringe techniques.

Thereafter, the solder is heated to a temperature at which the soldermetal becomes molten and a highly adherent fused solder bond is formed.For this purpose, vapor phase soldering is a preferred method, althoughother methods of heating the solder such as the use of belt furnaces,and infra-red heating can be employed. Any atmosphere for heating can beused, e.g., air, or an inert atmosphere employing an inert gas such asnitrogen.

As previously noted, the soldering operation employing the solderingcomposition of the invention is preferably utilized in a vapor phasesoldering operation utilizing a second fluorinated compound such asperfluorotriamyl amine as the heating medium and atmosphere.

The following table shows examples of soldering compositions accordingto the invention.

                  TABLE                                                           ______________________________________                                        COMPOSITIONS                                                                              A        B      C      D    E                                     ______________________________________                                        FC71 (vehicle)                                                                            14       13     12     13.5 11                                    (weight %)                                                                    Metal (40-70 micron                                                                       86       87     88     86.5 89                                    particle size)                                                                (weight %)                                                                    Lead        36       63     36     63   36                                    Tin         62       37     62     37   62                                    Silver      2        --     2      --   2                                     Bromocyclohexane                                                                          --       0.5    0.2    0.3  0.6                                   (wt. % of total                                                               composition)                                                                  Isopropyl alcohol                                                                         --       5      2      3    5                                     (wt. % of total                                                               composition)                                                                  ______________________________________                                    

The following are specific examples of practice of the invention:

EXAMPLE I

In producing solder composition or solder cream A of the above Table, alead-tin-silver metallic mixture in the proportions of 36% lead, 62% tinand 2% silver, is formed into a very finely divided powder as describedin detail above.

The metallic powder is blown into a chamber, the metal balls arescreenned so that the particle size thereof is between 40 and 70microns, and the particles are then mixed with the FC71 vehicle, suchoperations being carried out in an inert atmosphere, e.g., of nitrogen,to aid in substantially reducing oxidation of the metals. In thisexample, the vehicle and metal mixture are combined in a percentage of14% of the vehicle and 86% of the metallic mixture, by weight.

The solder cream is used to secure miniature electronic components to aceramic substrate. A printed circuit is produced on the substrate. Thecircuit is on one side of the substrate, with pads composed of platinumand gold being provided in certain areas, other parts of the circuitbeing covered with an insulating material.

The screen is placed over the circuit and the pads, and the above soldercream is screened onto the pads on the circuit board. Thereafter,electronic components including capacitors and resistors are carefullypositioned with their peripheral contacts on the solder cream-coatedpads.

The circuit board with the electronic components temporarily positionedand retained thereon by the solder cream, is placed in a chamber of avapor phase reflow system containing liquid perfluorotriamyl amine(FC70) in the bottom of the chamber. The FC70 liquid is then heated bysuitable means such as electric heating coils to a temperature of 215°C., producing boiling and vaporization of the FC70, the vapors ascendinginto the upper portion of the chamber and heating the ceramic substrateand the solder cream thereon. This results first in the melting andliquefaction of the perfluorotrihexyl amine (FC71) vehicle of the soldercream, which drains off the ceramic substrate into the liquid FC70 inthe bottom of the chamber, and dissolving in such liquid. Since themetal mixture or alloy employed in the solder cream has a melting rangebetween 177° C. and about 189° C., the system temperature melts themetallic particles in the solder cream and the solder alloy particlescoalesce to form a solder joint, fusing or adhering the contacts of theelectronic components to the pads on the circuit board, leaving thecomponents securely positioned in place and soldered to the pads on theboard.

Following the soldering operation, it is observed that all of thecontacts are securely soldered to the pads, and the solder joints areclean with no char or residue coating on the solder joints, electroniccomponents or the circuits. Hence, no special cleaning of the circuitboard is required following vapor phase soldering, to remove any suchdeleterious matter.

Any unreacted activator removed from the solder cream during vapor phasesoldering and passing into the liquid FC70 is insoluble therein andsettles in the bottom of the soldering system, together with smallamounts of tin and lead bromides and oxides. Such insoluble matter iscontinuously removed from the machine by filtration. During operation,periodically a portion of the FC70 fluid is removed and subjected todistillation, to separate the dissolved FC71 from the FC70.

EXAMPLE II

The vapor phase soldering procedure of Example I is followed employingrespectively, solder compositions B through E of the above Table.

Substantially the same results are obtained, namely, effective solderingof the electronic components occurred, with no formation of vehicleresidue on the solder joints, electronic components or circuits.

From the foregoing, it is seen that the invention provides a novelsolder composition employing a fluxless or rosin-free vehicle in theform of an organic fluorinated amine, particularly perfluorotrihexylamine, which results in clean solder joints following vapor phasesoldering, with essentially no deposition of organic residues, andavoiding the necessity of removal of any such residues. The abovefluorinated amine has advantageous properties similar to conventionalrosin fluxes such as pine rosin, while avoiding the residue formingdisadvantages thereof. The fluorinated amine vehicle can be readilyblended with particulate solder alloys into a solder cream or pastewhich permits stencilling intricate patterns on substrates to serve assolder pads. Moreover, the fluorinated amine vehicle of the solder creamof the invention is compatible with and dissolves in theperfluorotriamyl amine (FC70), a closely related fluorinated fluid,commonly used as the heating medium in soft-solder vapor phasesoldering.

While particular eabodiments of the invention have been described forpurposes of illustration, it will be understood that various changes andmodifications within the spirit of the invention can be made, and theinvention is not to be taken as limited except by the scope of theappended claims.

What is claimed is:
 1. A process for vapor phase soldering of electroniccomponents to a substrate, which comprisesapplying a solder cream to thesurface of said substrate or to the surface of components on saidsubstrate, said solder cream comprising 85 to 90% of finely dividedsolder metal and 10 to 15% of a fluorinated tertiary alkylaminecontaining from 4 to 8 carbon atoms, by weight of the sum of the twocomponents, and which alkylamine is semi-solid at room temperature andwhich confers a creamy consistency on the composition at roomtemperature, positioning electronic components on said substrate, withcontacts of such components on the solder cream-coated surface, placingsaid substrate in a chamber in contact with the boiling vapors of asecond liquid fluorinated tertiary alkylamine in said chamber, having alower boiling point than the fluorinated tertiary alkylamine in saidsolder cream, melting the solder metal in said solder cream and fusingthe solder joints of said electronic components to form solder jointssubstantially free of organic residue, and dissolving the fluorinatedtertiary alkylamine in said solder cream in the liquid fluorinatedtertiary alkylamine of lower boiling point in said chamber.
 2. Theprocess for claim 1, including the step of separating and removing thehigher boiling point fluorinated tertiary alkylamine dissolved in thesecond lower boiling point fluorinated tertiary alkylamine, bydistillation. PG,24
 3. The process of claim 1, wherein said fluorinatedtertiary alkylamine of said solder cream is a perfluortrihexyl amine andthe liquid fluorinated tertiary alkylamine in said chamber isperfluorotriamyl amine.
 4. The process of claim 1, including liquefyingthe fluorinated tertiary alkylamine in said solder cream by contactthereof with the boiling vapors of said second fluorinated tertiaryalkylamine and draining said liquefied fluorinated tertiary alkylamineinto the second fluorinated tertiary alkylamine of lower boiling pointin said chamber.
 5. The process of claim 1, including metal pads mountedon said substrate, said solder cream applied to said pads, the contactsof said electronic components positioned on the solder cream-coatedpads.
 6. The process of claim 1, said perfluorinated trialkyl aminebeing a perfluortrihexyl amine selected from the group consisting of thestraight chain and branched chain isomers, and mixtures thereof.
 7. Theprocess of claim 1, the second liquid fluorinated tertiary alkylamine insaid chamber being perfluorotriamyl amine.
 8. The process of claim 1,said solder composition including about 0.1 to about 1% of a brominatedactivator selected from the group consisting of bromocyclohexane andbromocampho, by weight of the composition, and about 2 to about 15 partsof a solvent to disperse said activator in the solder composition, perpart of activator, by weight.
 9. A process for vapor phase soldering ofelectronic components to a substrate, which comprises:applying a soldercream to the surface of said substrate or to the surface of componentson said substrate, said solder cream comprising 85 to 90% of finelydivided solder metal and 10 to 15% of a perfluorohexyl amine, by weightof the sum of the two components, positioning electronic components onsaid substrate, with contacts of such components on the soldercream-coated surface, placing said substrate in a chamber containingliquid perfluorotriamyl amine in the bottom of said chamber, heatingsaid liquid perfluorotriamyl amine to generate boiling vapors of saidperfluorotriamyl amine ascending into said chamber into contact withsaid substrate, melting and liquefying said perfluorotrihexyl amine insaid solder cream, said liquefied perfluorotrihexyl amine draining offsaid substrate into said liquid perfluorotriamyl amine in the bottom ofsaid chamber and dissolving therein, and melting the solder metal insaid solder cream and fusing the solder joints of said electroniccomponents to form solder joints substantially free of organic residue.10. The process of claim 9, including a printed circuit on saidsubstrate, metal pads mounted on said substrate in preselected areas ofsaid circuit, said solder cream applied to the surface of said pads, thecontacts of said electronic components positioned on the soldercream-coated pads, said solder joints adhering said contacts to saidpads.
 11. The process of claim 9, including the step of subjecting theliquid perfluorotriamyl amine containing the perfluorotrihexyl aminedissolved therein to distillation to separate and distill off theperfluorotriamyl amine, and recovering the higher boiling pointperfluorotrihexyl amine.